High Definition Television (HDTV) has become both a technical opportunity and a political problem. Current television technology in the United States is limited by a broadcast standard that is more than 30 years old. Technology has improved significantly and the opportunity is available for a new standard with far greater sound and picture quality. Improved picture quality generally requires more picture detail, more picture detail generally requires more transmitted information, and more transmitted information generally requires more bandwidth. Many HDTV systems have been proposed using channels with bandwidths of 12 to 30 Mhz.
NTSC, the current U.S. television broadcast standard, limits television signals to 6 Mhz channels. In order to allow larger bandwidth channels for HDTV, the Federal Communications Commission (FCC) would need to either allocate new bandwidth for HDTV or reallocate the current television spectrum. Many new services such as cellular telephone, pagers, and data networks are clamoring for more bandwidth. Allocating more bandwidth for television would necessarily mean less bandwidth for these other important services.
Reallocating the current spectrum would have an adverse impact on existing television receivers. The FCC has made it clear that it is not willing to accept either the allocations of new bandwidth for television transmission nor the reallocation of the television spectrum. Any new HDTV standard must, therefore, be compatible with the current allocation of 6 mhz television channels.
The MUSE system developed by NHK of Japan addresses the problem of available bandwidth by broadcasting HDTV signals direct from satellites at very high frequencies. This adds the cost of a satellite earth station to the already high price of a HDTV receiver and is, therefore, an economically unacceptable solution in the U.S.
Another limiting factor of current television broadcast spectrum is co-channel interference (interference between broadcasters using the same channel in neighboring cities). As an example, Chicago broadcasts on channels 2, 5, 7, 9, and 11 in the VHF band while Milwaukee broadcasts on channels 3, 6, 8, and 12 in the VHF band. These neighboring cities are not allowed to broadcast on co-channels.
Due to the large power needed to broadcast current NTSC signals, signals from Chicago broadcasts may carry well into the Milwaukee service area just as signals from Milwaukee may travel well into the Chicago service area. While these signals are normally not strong enough to provide an acceptable picture in the neighboring service area, they can present sufficient interference making it necessary to restrict the use of co-channels by the two neighboring service areas. A system that allows HDTV to broadcast on these previously "taboo" co-channels will be able to provide HDTV service without any hindrance to existing NTSC broadcasts or visa versa.
The '956 application discloses a method for transmitting a television signal comprising removing the low frequencies from the signal, digitizing the low frequency information and broadcasting that low frequency information in the vertical blanking interval of the video signal. Since, most of the power required to broadcast a television signal is used to broadcast the low frequencies, the transmission system disclosed in the '956 application requires far less power than a conventional system. As a result of the reduced broadcast power, a television signal can be broadcast on previously taboo channels without interfering into a co-channel of a neighboring service area.
The amount of low frequency information or low frequency bandwidth that can be removed is determined by the amount of digital information that can be inserted in the vertical blanking interval. More efficient data compression of the low frequency information will result in both less power required to transmit the signal and more bandwidth remaining to transmit more detail in the high frequency component.